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Animation - IR |
 Reading temp/humidity trace |
Animation - Reading SJ phase |  Animation - Reading WCB phase
 Animation - time of Reading max SJ gust
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Part of Sting Jet conceptual model |
Damage Swathe conceptual model |
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4. Predictability (including 40r1 E-suite evaluation)
4.1 HRES
4.1.1 O-Suite
Strip Sequence of forecasts below (left columnand second left columns) shows 48h maximum gusts from successive HRES forecasts from DT 00UTC 21st through to 00UTC 27th for VT period 06UTC 27th to 06UTC 29th (on 12up image left click, then right click -> view image to get full resolution), for VT period 06UTC 27th to 06UTC 29th. Note how the early forecasts jumped around, as one might expect. Later on very extreme gusts were forecast for areas SW of the UK, and as the event approached this zone of extreme gusts migrated its way eastwards, which is perhaps not what one would want/expect. End of period mslp is also shown.
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Mean wind speeds and wind gusts are not just dependant on low depth. The multiple possible scenarios provided by HRES and ENS, in O- and E-suites, have shown considerable sensitivity, in different parts of the storm - that make the difference between a normal windstorm, and a once-in-a-lifetime event. Pinning this down is the real challenge. The following plots illustrate some of the issues - they have been extracted from animations in Ervin's daily reports, linked to above.
The top row in each table cell is from the O-suite; the bottom row from the E-suite.
The left table column shows gusts, the right mean 10m winds.
Validity times are 03 (top row of table), 04 (middle) and 05UTC (bottom), with gusts being the maximum in the hour up to that time.
| Max 10m gusts (in preceding hour) - Obs on right, same colour scale | 10m mean speed - Obs on right, same colour scale (but different to gust scale) | |||
03UTC O-suite at top E-suite below
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|---|---|---|---|---|
| Max 10m gusts (in preceding hour) | 10m mean speed | |||
| 03UTC |  |  | ||
04UTC O-suite at top E-suite below |  |  | ||
05UTC O-suite at top E-suite below |  |  | ||
06UTC O-suite at top E-suite below |  |  | ||
07UTC O-suite at top E-suite below |  |  | ||
| Gust scale: 20-green-24- | Gust scale: 20-green-24-26-28-dark blue-30-orange-32-red-34-magenta-37-40 (m/s) |
First thing Several things to note:
In the warm sector (E and SE of low centre)
1. Huge variability in gusts is the huge variability in the warm sector due E of the low centre in both suites, variability which yet only minor variability in mean speeds. Gust variability does not appear to relate to geostrophic or gradient winds, both of which look, to the eye, to be relatively unvarying , and also much the same in 0- and E-suites in this region.
Next note how the gusts jump around between successive times - eg in the E-suite we go from locally 30m/s over Kent on the first frame, to widely only 25m/s two hours later. Gusts over the sea also appear to be reducing markedly in that time frame, despite no clear reduction in gradients.
with time (consistent with the steadier mean speeds).
2. Much lower gusts overall in the E-suite, despite what appear to be mean winds that are very similar to the O-suite.
3. Considerable jumpiness in gusts between successive times - eg in the E-suite we go from locally 30m/s over Kent 02-03UTC, to widely only 25m/s 04-05UTC. E-suite gusts over the sea also reduce markedly 02-03 to 03-04UTC.
4. Compared to OBS, both suites look to have overdone the gusts, but the E-suite seems much better (lighter).
Sting jet area (SW of low centre)
1. Starts to appear in O-suite, correctly, by 05UTC, but not in E-suite.
2. In the O-suite sting jet the gust increase nicely mirrors the mean speed increase (allowing for rapid translation eastwards during the 1h interval).
3. Compared to OBS the O-suite representation is good (though gusts slightly underdone perhaps). The E-suite gusts are much worse (due to no sting jet).
So we have identified very different behaviours according to synoptic location. In general terms the model behaviour also nicely reflects behaviour seen on the Reading observation graphs above. Note the isolated 25 m/s warm sector gust, unrelated to a change in the mean, as discussed above, and a sting jet gust max later that was related to the mean. One could imagine that the isolated warm sector gusts could have been hard to predict, and in this sense the volatility of the gust parametrisation in this regime in both suites may not have been that surprising. Conversely the sting jet gusts, which relate much more clearly to pressure gradients and mean winds, are on the one hand easier to predicte because of this, but on the other very hard to predict because those pressure gradient details themselves pose considerable difficulties for the models.
As regards the source of the large gust differences between E- and O-suites, these must relate to the gust parametrisation scheme, which contains 3 terms, a) the 10m mean wind, b) a turbulence factor, related to stability and c) a convective gust adjustment. The latter is only activated if the deep convection parametrisation is active (though this does not have to be surface triggered). It would seem that contributions from (c) or more likely (b) are making the E-suite gusts less. Vertical diffusion has changed in the E-suite and this may be relevant; this is now under investigation within Physical Aspects.
4.2 ENS
The sequence of figures above shows the strike probability of cyclone with maximum wind speed (mean) over 60 kt (31 m/s) at 1 km height. The plots are valid for the 28 October (24-hours). The first forecast is from 28 Oct 0 UTC and one day is added for each plot. With longer lead time the feature is delayed (too slow) in the model, but the path of the cyclone seems very consistent up to 6 days before the landfall.
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